World population reached 7.2 billion in mid-2013, according to United Nations demographers, with present and projected future growth propelled in part by unexpectedly high fertility in a number of developing countries. Based on current trends in global birth, death, and migration rates, the UN Populations Division projects a variety of future scenarios, with the three principal ones suggesting that world population will be somewhere between 6.8 billion and 16.6 billion at the end of this century. In the latest Vital Signs Online trend, I discussthese latest projections and what they mean for the environment.

The UN demographers determined that 82.1 million people were added to the world’s population in 2012—the highest annual increment since 1994. The new report, World Population Prospects: The 2012 Revision, dispels a widespread expectation that population growth would end “on its own” sometime in the second half of the twenty-first century. Rather, the new medium-fertility or best-guess scenario suggests the most likely outcome is that world could gain more than 10 million people in the year 2100 and close the century at 10.9 billion. The new projections suggest a global population of 9.6 billion by 2050—about 700 million people more than the 8.9 billion the UN Population Division had projected for 2050 just 10 years ago.

Although the biggest surprise in the report came from the projections of faster future population growth than had been expected, these numbers actually have their roots in a surprise about the present: Women in many developing countries are having more children today than UN demographers previously thought. Indeed, the UN authors reported that they had raised by a full 5 percent their estimates of current fertility in 15 sub-Saharan African countries—including Nigeria, Niger, Ethiopia, and the Congo—where family size is already among the highest in the world.

Although the reasons behind the higher-than-expected fertility in many countries are not fully understood, they correlate well with recent government reluctance to give priority to and fund family planning services in some of the world’s poorest countries. Spending on family planning services in developing countries by governments, wealthier donor governments and intergovernmental agencies has stagnated in recent years at around $4 billion annually. More than twice that is needed to reach the estimated 222 million women who are sexually active and do not want to become pregnant but are not using contraception. About two out of five pregnancies worldwide are unintended—in industrial countries as well as developing ones—and more than one in five births worldwide results from such pregnancies.

On perhaps the most positive note in the new projections, UN Population Division demographers believe that every country in the world is currently experiencing a longer life expectancy in the 2010-to-2015 period than between 2000 and 2010. They project continued improvement in life expectancy throughout the century, when all the new projection scenarios agree that life expectancy for the world will average 82 years, up from 70 years today.

Yet this rosy assessment of global longevity nine decades from now does not take into account changing environmental conditions worldwide. The UN demographers, like others who produce major population projections, decline to factor in the possibility that mortality trends will vary from recent history, making no mention of possible downward shifts in life expectancy due to climate change or any other environmental impacts of human activities.

Further highlights from the report:

The new report estimates that the world’s population is growing at about 1.15 percent annually, and—despite the higher-than-anticipated fertility in many countries—that the growth rate is continuing to slow.

Most human beings—an even 60 percent—live in Asia, with Africa the second most populous region, followed by Europe, Latin America and the Caribbean, North America, and Oceania.

Approximately 96 percent of the growth is occurring in developing countries, with Asia accounting for 54 percent of that growth.

The new UN “most likely” population projection foresees not just a long-lived human population of 10.9 billion in 2100 but one of 4.2 billion in Africa with a life expectancy of 77 years compared with 58 today.